Update magic-commands.md with #!set

[marckruzik]

#!set should be listed somewhere in the magic commands page.
Maybe it's not the right place in the page, so please correct it if needed.

[marckruzik]

I changed the description of #!set.

I also used the same way to express the description of sub parts (including xxx) (with parentheses) to improve description consistency. As there are no writing rules for consistency, I am just editing it here as I saw it. Better have the same style in a page, rather than different ones.